Technical Field
The present invention relates to a toner used in an image forming apparatus using an electrostatic duplication process such as copiers, facsimiles and printers.
Description of the Related Art
As a method of preparing a toner, besides a conventional kneading and pulverizing method, a chemical toner method such as a suspension method and an emulsification method using an organic solvent and an aqueous medium, a suspension polymerization method controlling and polymerizing a polymerizable monomer drop to direction form a toner, and an agglutination method agglutinating emulsified particles to form a toner is known.
As one of the chemical toners, a core/shell toner formed of a core including a resin advantageously used for heat fixation and covered with a resin advantageously used for charging and heat resistance is known.
Among them, in consideration of bleed-out of a release agent when a toner is fixed, Japanese published unexamined application No. JP-2011-095286-A discloses a toner having a partially-coated shell in the shape of a projection, and Japanese published unexamined application No. JP-2011-046865-A discloses a core shell toner having high adherence of the core to a resin.
A toner typically includes a wax to prevent offset when fixed.
When a toner includes a large amount of a wax to further improve offset prevention in high-speed printing, the wax is poorly dispersed, resulting in not only insufficient fixability, but also various problems such as poor developability, durability and preservation stability.
Japanese published unexamined application No. JP-2006-301093-A discloses an image forming method using a toner having a wax concentration of from 0.02 to 0.70 mg/cm3 in a resultant extraction liquid after dispersed in n-hexane at 23° C. and a concentration of 15 mg/cm3 and extracted for 1 min, and a specific fixer.
When the wax concentration is not less than 0.02 mg/cm3, at least a part of the wax is uniformly dispersed in a binder resin of a toner on the molecular level, and wax particles and wax domains largely decrease in the toner.
In Japanese published unexamined application No. JP-2006-301093-A, aliphatic hydrocarbon wax is preferably used, and paraffin wax is more preferably used. However, non-polar wax such as paraffin wax is comparatively difficult to uniformly disperse in polyester resin. Further, when the wax is finely dispersed on the molecular level, the resultant toner noticeably deteriorates in releasability. Rather, when large wax domains are dispersed, only the wax exudes and the resultant toner improves in releasability.
Conventionally, when the wax in a toner is increased, an amount thereof exposed on the surface thereof is increased at the same time, resulting in problems such as contaminated development. When the exposed amount is limited with a wax dispersion aid or by controlling a polarity of the binder resin, the wax working when the toner is fixed is decreased, resulting in insufficient offset prevention. Further, when a non-polar wax having a low melt viscosity is used to prevent offset, the non-polar wax has too low compatibility with a binder resin having high polarity to disperse, resulting in difficulty in controlling the exposed amount of the wax.
Because of these reasons, a need exists for a toner limiting a wax exposed on the surface thereof, preventing offset even when including a large amount of a wax, and having optimal fixability.